1. Field of the Invention
The present invention relates to a bottom machine for a glass processing device for manufacturing glass containers made from a glass tube, comprising one or a plurality of holding units for holding the glass tube or glass container, with the holding units being mounted so as to rotate around their own axis and around an axis of rotation of the bottom machine in order to convey the glass tube to various processing positions, a pressure source for supplying a flow of gas, and a duct system that communicates with the pressure source for directing the gas flow to the holding units and for feeding the gas flow into the glass tube or glass container. The invention further relates to a glass processing device as well as a method for manufacturing glass containers.
2. Description of Related Art
Glass containers that are manufactured using generic glass processing devices have a cylindrical section or conical section. In particular, generic glass processing devices are used to manufacture glass containers as syringes and vials, the syringes and vials being used primarily for storing and administering pharmaceutical products, such as medications.
U.S. Pat. No. 1,700,326 A, U.S. Pat. No. 2,193,376 and EP 0 293 661 A1 show devices in which glass containers are obtained from a glass melt by means of a blowing method. In this process, a specific volume of the glass melt is placed in a mold, the glass having a very high temperature (approximately 1600° C.). A cavity is then produced by, for example, inserting a mandrel into the molten glass. Afterwards, compressed air (several bars) is blown into the cavity, so that the glass presses against the walls of the mold and consequently is molded. In order to remove the resulting glass containers from the mold, it must be possible to open the mold. Conventionally, there are two parts of the mold that can be opened. However, in order to prevent deformation of the glass container when the mold is opened, the mold must be cooled, at least at the contact surface between glass and mold, to a temperature that affords the glass containers a certain degree of form stability (approximately 600° C.). Therefore, it must be possible to cool the mold, which can also be conducted by way of the compressed air. In addition, it must be possible to remove the glass container from the mold by using tongs, for example, so that there must be a certain degree of form stability before the glass container can be removed from the mold. In general, glass containers that are manufactured using the blowing method are suitable for storing pharmaceutical products. However, the glass containers resulting from the blowing method have a wall thickness that varies greatly. On account of the optical distortions resulting from this, automated product inspection is possible to only a very limited extent.
DE 103 41 300 B3 shows a generic glass processing device for manufacturing glass containers from a glass tube; however, in this case, no air or other gas can be fed into the glass tube.
Glass processing devices of the kind mentioned in the beginning have a parent machine and a bottom machine, both of which can rotate and have a number of holding units, which comprise a clamp chuck, for example. Usually, the bottom machine is arranged below the parent machine. A glass tube that is approximately 1.5 m in length is clamped in the clamp chuck of the parent machine, the glass tube protruding downward from the clamp chuck by a specific length. At its downward protruding open end, the glass tube undergoes certain processing operations, which are performed at different processing positions. To this end, the parent machine and, together with it, the clamp chuck are rotated by a certain angle from one processing position to the next. If the downward protruding end of the glass tube has been completely processed, so that, for example, it has a rolled edge or a thread, then a part of the glass tube has to be severed. The corresponding clamp chuck of the parent machine is aligned with a clamp chuck of a holding unit of the bottom machine at the processing position at which the severing step is performed. The holding unit of the bottom machine can move axially and grasps the glass tube somewhat above the downward protruding end of the glass tube. In the region between the clamp chuck of the parent machine and the clamp chuck of the bottom machine, a gas burner is usually directed at the glass tube in order to heat it, with it being possible to heat the glass tube in other ways as well. In this case, the gas burner is placed in a fixed position. In order to heat the glass tube in a manner that is as rotationally symmetric as possible, the holding units can rotate around their own axis. As a result of rotating the glass tube, it is heated not only at one site by the gas burner, but the heating is distributed uniformly over the circumference. The glass tube is heated until it is sufficiently viscous that the part that is clamped in the clamp chuck of the bottom machine can be severed from the remaining part of the glass tube by lowering the clamp chuck of the bottom machine. In the process, the heated part of the glass tube tapers and constricts to such an extent that a closed bottom forms at the severing site, one at the drawn-off part situated in the clamp chuck of the bottom machine and one at the part of the glass tube remaining in the clamp chuck of the parent machine.
In the following, the part that is situated in the clamp chuck of the bottom machine will be considered. As already discussed, the free open end is already completely processed, but the bottom of the now resulting glass container is not yet in its desired form. In this stage of processing, the bottom of the glass container is situated above the open end in relation to the direction of action of gravity, this entailing the following: As described above, the bottom is produced by a thermal severing step, so that the viscosity of the glass in the bottom region is still so high that the bottom sags more or less strongly downward depending on the diameter of the glass tube used. In addition, the bottom exhibits a radially changing wall thickness. In order to be able to furnish the bottom of the glass container with the desired characteristics, it needs to undergo yet further, primarily thermal processing steps. In order to counteract the sagging of the bottom, a gas, usually air, is blown into the glass container through the open end of the glass container, as a result of which a back pressure that supports the bottom is created. Depending on which processing steps remain to be performed, a sufficient amount of gas is blown in to cause the bottom to bulge upward, as a result of which the accessibility of the bottom to processing tools, such as gas burners, is increased. Finally, the glass container is brought to the desired length, this being accomplished by pressing the bottom against a bottom template. To this end, a gas flow is fed into the glass container, resulting in the creation of back pressure when it meets the bottom and pressing the bottom against the bottom template. However, because the glass container is open, no significant static overpressure is created in the glass container. Both the static overpressure and the back pressure amount to no more than 1 mbar.
Blowing in a gas flow has yet another aspect. Borosilicate glasses are preferably used for storing pharmaceutical products, because they offer high hydrolytic resistance at relatively low cost. Borosilicate glasses also contain sodium to lower their melting point. However, Na ions are not bound valently in the glass, but rather migrate through the glass matrix, which is defined primarily by SiO2. If the glass tube is heated to sever it, temperatures of greater than 1200° C. are necessary, such temperatures clearly lying above the vaporization temperature of sodium. Consequently, large amounts of Na vaporize from the bottom and deposit once again at various sites in the vial. The vaporization of sodium also has the effect that boron is entrained in the form of borates and also vaporizes, even though the borates, in comparison to sodium, are markedly more strongly bound in the glass matrix.
The glass container is thermally treated very strongly at the bottom, whereas, at the cylindrical section, no thermal treatment takes place. As a result of this, strong thermal stresses are created in the glass container, leading to potential cracking of the glass container after cooling. In order to prevent this, a further thermal treatment needs to be performed so as to relieve the stresses. This thermal treatment takes place at approximately 600° C., that is, clearly below the temperatures that are necessary for severing the glass container from the glass tube. In the process, a large part of the sodium borate vaporizes. The part that does not vaporize bakes into the walls of the glass container. This poses a problem in that Na ions can migrate into the substance stored in the glass containers. Particularly in the case of pharmaceutical substances, this is undesirable. The migration tendency strongly depends on the substances being stored in the glass containers and the pH value thereof. As a result of introducing the gas flow into the glass container or into the glass tube, part of the sodium is also removed from the glass container during the severing process, so that less sodium is able to bake into the walls. In this context, the tendency for sodium to be able to migrate into substances being stored in the glass containers is also referred to as surface alkalinity, which can also be reduced by blowing in gas in a undefined manner.
Blowing air into the glass container or into the glass tube is conducted in known glass processing devices as follows: Below the clamp chuck of the bottom machine, a tube runs parallel to the axis of rotation of the bottom machine, with the outlet opening of the tube being situated directly below the open end of the glass tube or glass container. Placed along the circular path traveled by the clamp chuck of the bottom machine is a correspondingly curved bottom tube furnished with a slot or a number of holes, which are arranged on the top surface. The inlet opening of the tube running parallel to the axis of rotation of the bottom machine is situated at a specific distance above the bottom tube. The bottom tube is charged with gas, usually air, which leaves the slot or the holes and enters the respective inlet opening. As mentioned at the beginning, the holding units or the clamp chucks can be rotated for uniform heating of the glass tube or glass container. The tube running parallel to the axis of rotation of the bottom machine rotates along with them.
The holes are unprotected toward the top, so that they can become quickly plugged by glass splinters, oil, and other particles that are present in the harsh surroundings of the glass processing machine. Consequently, the flow conditions change in the region between the bottom tube and the inlet opening, so that it is nearly impossible to feed a reproducible gas flow from the pressure source into the glass container or glass tube. It can never be known what volume flow actually enters the glass container or glass tube. It is noted at this point that it depends on the progress of the processing whether the gas still is being fed into the glass tube or else into the already existing glass container with closed bottom. Blowing is performed regardless thereof.
Not only sodium borates, but alkali borates in general have yet another detrimental property. The rate of vaporization of alkali borates increases exponentially with increasing temperature. When the bottom of the glass container is processed, sodium borate vaporizes out of the bottom region and deposits once again in a condensation zone on the walls of the glass container. The glass is already relatively cold in the condensation zone. However, an inward diffusion zone forms between the bottom and the condensation zone, in which more sodium borate diffuses into the glass than vaporizes. Consequently, there results an enrichment of sodium borate in the inward diffusion zone and a depletion in the bottom, with the depletion in the bottom having no negative consequences. However, the enrichment of borate in the inward diffusion zone has the following consequences:
In the near-surface region of the inward diffusion zone (approximately 30 to 200 μm from the inner surface of the glass container), the enrichment of boron has the effect that the borosilicate glass, composed primarily of Si, Na, and B, is no longer miscible after cooling, because there is a miscibility gap in the ternary phase diagram here. Consequently, two phases of different composition are formed, which necessarily also have different chemical and physical properties. One of the two phases also exhibits a lower hydrolytic resistance, so that it is more readily attacked, resulting in stresses in the near-surface region of the inward diffusion zone. As a result of this, particle-shaped glass components detach from the surface, these components having a clearly smaller dimension in one axis than in a plane perpendicular to this axis. These particles then enter the substance stored in the glass container, this having particularly great consequences when the substance is administered as a medicine. The tendency for detachment of these flaky particles is also referred to as the delamination tendency.
In known glass processing devices, the machine operator in charge adjusts the magnitude of the gas flow on the basis of his experience, such that the bottom obtains the desired geometric properties and the limit values set for surface alkalinity are not exceeded. Another machine operator can achieve the same geometric properties of the bottom and the desired values of surface alkalinity with an entirely different magnitude of the gas flow. Reproducibility is not afforded. However, the delamination tendency cannot be reliably lowered into uncritical ranges with known glass processing devices and, for a long time, was not the focus of manufacturers of glass containers for the above-mentioned purposes.